A detecting element of the prior art for detecting rotation of a gear is a magnetic coil 102 coupled with a bias magnet 103, as shown in FIG. 6, so as to gain an induced voltage across the coil by detecting a variation of magnetic flux density due to a change of magneto-resistance produced by projections and depressions of a rotating gear 101. In this instance, the rotating gear 101, the bias magnet 103 and the magnetic coil 102 are considered as a generator, and a magnitude of the induced voltage depends upon a rotational speed, and a signal is detectable with two lines. Although it is a simple and inexpensive structure, it has a fundamental problem in that the induced voltage decreases at low speed.
It is also known that a ferromagnetic magneto-resistance element 104 is used as a detecting element in combination with a signal processing circuit 105 for superposing a rectangular wave current on a power supply line by a constant-current drive, as shown in FIG. 7. It utilizes a pair of current-miller circuits for the constant-current drive, in which a current of one of the current-miller circuits is kept flown at all the time, while a current of the other current-miller circuit is turned on and off by an output signal at a midpoint of the ferromagnetic magneto-resistance element 104 of a half bridge, so as to produce a rectangular wave current, and superposes the rectangular wave current on the power supply line in order to reduce a signal output line and to convert three lines into two lines.
In recent years, a performance required of a rotational sensor for vehicle installation is an ability to detect rotation at an extremely low speed.
Detecting elements suitable for this purpose are so-called magneto-electric elements of positional detection type, such as a Hall element, a ferromagnetic magneto-resistance element, and a semiconductor magneto-resistance element. The semiconductor magneto-resistance element, in particular, is one of the most suitable for detecting a rotational gear, since it produces a high detection signal voltage. If a magneto-electric element of positional detection type is used for the rotational sensor, two current supply lines and one signal output line, for a sum of three lines are generally needed to constitute the circuit. A rotational sensor capable of detecting an extremely low speed with two lines is desirable, however, in consideration of compatibility with a magnetic coil, reduction of a vehicle weight, effective usage of a cab space, etc. A two-line system in a rotational speed-detecting device of the positional detection type detects a variation of magnetic leakage flux produced by a rotating or moving body with a magneto-electric element, and superposes a rectangular wave current on the power supply line after signal processing, so as to reduce a number of a signal output line by way of calculating the number of rotations or the amount of movement of the rotating or moving body with a computer by detecting a voltage across a resistor inserted in the power supply line at a power supply terminal. This method requires to reduce absolute values and a relative difference of a high current value and a low current value of the rectangular wave as small as possible, in order to reduce a power consumption of the entire circuit and to hold down heat generation of the circuit elements as little as possible. On the other hand, the smaller it is to keep an absolute current value of the rectangular wave current, the more important a preciseness of the current value of the rectangular current waveform is, in order to reliably recognize the high current value and the low current value of the rectangular wave. For this reason, a method has been used in the past in which a current consumption of an entire signal processing circuit is controlled by using a constant-current circuit.
In the rotational sensor, if a semiconductor magneto-resistance element (hereinafter referred to as "SMR") is used for the detection unit, as in the past, the fluctuation of resistance value is so large due to temperature, e.g., a variation in an extent of approximately 50 times in the resistance value within a range of -40.degree. C. to 150.degree. C., although an output is high under a condition of strong magnetic field. Therefore, the following problems arise when it is driven with a constant current.
(1) Since a resistance value of the SMR is low at high temperature, a detecting output becomes lower at low resistance as compared to the detecting output at high resistance, considering a reduction of the magnetic sensitivity by approximately 30%, coupled with a decrease in the amount of change in the resistance. Therefore, it falls into a state wherein a comparator is unable to provide an on-and-off operation taking into consideration a drift of the circuit elements, a hysteresis, and a temperature drift at a midpoint of the magneto-electric element.
(2) If the SMR and a comparison voltage generating unit are driven together with a constant current under the normal temperature condition, a noise margin becomes smaller, since a voltage across the SMR constituting a half bridge changes due to a variation in resistance value of the SMR caused by its rotation, which also leads to a change in a midpoint voltage of a comparison voltage resistor.
Also, there has been a problem of an increase in the cost with ferromagnetic magneto-resistance elements (hereinafter referred to as "MR") if it is used for the detecting unit in a rotational sensor of the prior art, because it has a smaller rate of change of magneto-resistance than the SMR. It requires a power element for the circuit element as a current value of the rectangular wave current increases if a constant-current driving current is increased in order to obtain a large output. It requires more number of circuit elements in order to amplify output of the element output if the driving current is decreased, since the MR output becomes smaller as in case of the SMR, thereby resulting the comparator being unable to provide an on-and-off operation, taking into consideration a drift of the circuit elements, a hysteresis and a temperature drift at a midpoint of the magneto-electric elements.
Moreover, in either case of utilizing the SMR or the MR, there has been a problem in securing an accurate current value for the rectangular wave current and making the circuit structure as simple as possible at the same time over a variational range of the power supply voltage and a variational range of servicing temperature, with an indispensable condition of reduction in both size and cost, in consideration of compatibility with a magnetic coil.